USB: dwc3: core.c: Add SW workaround for DRD device buffer overflow issue

When Host sends Vendor Device test request to device, USB core writes
Vendor device test event into event buffer which might cause buffer
overflow due to HW bug. This could lead to memory corruption. Hence
allocate 8 bytes more for event buffer than reported to controller
through GEVNTSIZE register.

CRs-Fixed: 493717
Change-Id: I42cf3af709021ec201288aec9cca5f3af473bf3c
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>